Mr. Backlot's Trophy Garden
Mr. Backlot is a very wealthy man living in Sinnoh, residing in a mansion that can be found on Route 212, the Hearthome City side. There are several things to do here including, most importantly, catching some rare Pokémon. As well, perhaps not coincidentally, the Pokémon found in his Trophy Garden can be used to gain entrance to Hearthome City's Amity Square.
There is a book on the table in Mr. Backlots' room. If you examine the book it will give you the PokéDex entry for Manaphy. If you do not have a Manaphy that you can trade over this will be important in helping you complete your SinnohDex as Manaphy is a requirement for its completion.
Well, I didn't make the name of the challenge up, but is fun to type... At any rate, if you head to the west wing of Mr. Backlot's you will meet a maid who will, once a day, proposition you for a challenge. This challenge can only be made in Platinum as in Diamond/Pearl you are denied access to the area.
The goal is simple. The maid will give you a number of turns you have to defeat all five maids in. If you can do this you will be asked to face one last maid.
The first five maids will use one Clefairy each. The first maid's Clefairy is at level 25. For each successive maid the level will increase by two levels. You will have to be lucky enough to avoid one of them using Endure.